Oxide superconductors have comparatively high critical temperatures; (e.g., about 90 K for an oxide superconductor of the YBa.sub.2 Cu.sub.3 O.sub.7-.delta. system, and about 105 K for the Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.8+.delta. system). Thin films or wires of such oxide superconductors have great potential for application in the electronics and energy fields.
Thin films of superconducting oxides have principally been produced onto single-crystal substrates of oxides such as MgO and SrTiO.sub.3 by vapor-phase processes including vacuum deposition and sputtering as described, e.g., in Electronic Ceramics, vol. 19, pages 50 to 56 (1988) (Japan). In addition to requiring a bulky and expensive apparatus, these methods are incapable of forming thin films of large area.
Recently, a method has been proposed, as described, e.g., in Nippon Seramikkusu Kyokai Gakujutsu Ronbunshi (Journal of the Ceramic Society of Japan), vol. 96, pages 417 to 420 (1988) and ditto, vol. 96, pages 450 to 454 (1988), in which viscous solutions of oxide superconductors such as the YBaCu.sub.3 O.sub.7-.delta. systems and the Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.8+.delta. systems are coated onto oxide substrates such as MgO, ZrO.sub.2 (Y. doped), SrTiO.sub.3 and Al.sub.2 O.sub.3 by spin coating, dip coating, screen printing or some other suitable method, followed by a heat treatment to form thin films. The viscous solutions of the oxide superconductors are prepared by conditioning uniform solutions having organometallic compounds and various salts of metals mixed and dissolved in suitable solvents. This approach is can be used to form thin films of oxide superconductors since not only it obviates the need to employ a bulky and expensive apparatus but it is also capable of easily forming thin films of large area.
However, the thin films formed by heating the coatings of the viscous solutions described above contain not only high-temperature superconducting phases having high critical temperatures (e.g., YBa.sub.2 Cu.sub.3 O.sub.7-.delta. and Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.8+.delta.), but also other phases such as those of the carbonates of starting metals (e.g., BaCO.sub.3 in the case of the YBa.sub.2 Cu.sub.3 O.sub.7-.delta. system, and SrCO.sub.3, CaCO.sub.3 and a low-temperature superconducting phase having a critical temperature of 85 K in the case of the Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.8+.delta. system). It has thus been impossible to produce a single phase superconducting thin film at a low processing temperature with consistent and reproducible results. This has been an obstacle to the efforts to commercialize the production and use of thin films of superconducting oxides.
Heating to temperatures of 750.degree. C. or higher is necessary to decompose carbonates such as BaCO.sub.3, SrCO.sub.3 and CaCO.sub.3. However, at such high temperatures, a reaction occurs between the substrate and the thin film of the superconducting oxide by interdiffusion, producing a boundary layer that differs in nature from either component. Furthermore, an oxygen deficiency results or difficulty is encountered in controlling the amount of oxygen. Thus, the superconducting characteristics (e.g., critical temperature and critical current density) of the thin film deteriorate due to such high temperature processing. Other disadvantages are associated with high-volume production and most notable is the inability to form thin films of uniform thickness. Therefore, one of the major problems in producing superconducting thin films by the solution process has been how to prevent the formation of carbonate salts.